U.S. patent number 5,226,417 [Application Number 07/667,152] was granted by the patent office on 1993-07-13 for apparatus for the detection of motion transients.
This patent grant is currently assigned to Nellcor, Inc.. Invention is credited to Deborah A. Briggs, Robert L. Clark, Adnan I. Merchant, David B. Swedlow, Jessica A. Warring.
United States Patent |
5,226,417 |
Swedlow , et al. |
July 13, 1993 |
Apparatus for the detection of motion transients
Abstract
An apparatus for detecting movement in patients coupled to pulse
oximeters and a method for using the signal generated by the
apparatus to filter out the effects of motion from the test results
generated by the pulse oximeter are disclosed. In a preferred
embodiment, a piezoelectric film located in close proximity to the
pulse oximeter's sensor provides a voltage signal whenever movement
occurs near the sensor. This voltage signal is processed and the
resulting signal is used to correct the oximeter's measurements. In
addition to piezoelectric film, accelerometers and strain gauges
are also usable to provide a signal indicative of motion.
Inventors: |
Swedlow; David B. (Foster City,
CA), Clark; Robert L. (Hayward, CA), Merchant; Adnan
I. (Fremont, CA), Briggs; Deborah A. (San Ramon, CA),
Warring; Jessica A. (Millbrae, CA) |
Assignee: |
Nellcor, Inc. (Hayward,
CA)
|
Family
ID: |
24677018 |
Appl.
No.: |
07/667,152 |
Filed: |
March 11, 1991 |
Current U.S.
Class: |
600/336; 356/41;
600/322 |
Current CPC
Class: |
A61B
5/721 (20130101); A61B 5/14552 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 5/11 (20060101); A61B
005/00 () |
Field of
Search: |
;128/633,664,665,670,677,682 ;356/40,41 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Nellcor pamphlet "Nellcor redefines pulse oximetry. Introducing the
Nellcor N-200 with ECG synchronization". .
Nellcor pamphlet "N-200. Nellcor N-200 pulse oximeter with C-LOCK
ECG synchronization". .
"C-LOCK ECG Synchronization Principles of Operation", Pulse
Oximetry Note Number 6, Reference Note, Nellcor Inc.,
1988..
|
Primary Examiner: Cohen; Lee S.
Assistant Examiner: Pontius; Kevin
Attorney, Agent or Firm: Townsend and Townsend Khourie and
Crew
Claims
What is claimed is:
1. A sensor for attaching to a patient for electro-optical
measurement of at least one blood characteristic, comprising:
optical signal means for generating a first electrical signal
indicative of the at least one characteristic of the blood in a
portion of the patient's tissue;
a piezoelectric film;
signal processing means, coupled to said piezoelectric film, for
generating a second electrical signal indicative of movement in and
of the portion of the patient's tissue; and
means for transmitting the first and second electrical signal to an
instrument for determining the blood characteristics.
2. The sensor of claim 1 wherein the signal processing means
comprises an electrical impedance means coupled to the
piezoelectric film.
3. The sensor of claim 1 wherein the signal processing means
further comprises an electrical impedance means coupled to the
piezoelectric film, the value of the electrical impedance means
indicating the geometry of the piezoelectric film.
4. A system for measuring a blood characteristic of a patient
comprising:
a sensor comprising:
optical means for generating a first electrical signal indicative
of a characteristic of the blood in a portion of the patient's
tissue;
a piezoelectric film;
signal processing means, coupled to said piezoelectric film, for
generating a second electrical signal indicative of movement in and
of the portion of the patient's tissue; and
means for transmitting the first and second electrical signals to
an instrument for determining a blood characteristic;
means for receiving the first and second electrical signals from
the sensor;
first processing means for operating on the second electrical
signal for generating a signal indicative of motion; and
second processing means for operating on the first electrical
signal and the signal generated by the first processing means for
determining a blood characteristic.
5. The system of claim 4 wherein the signal processing means
comprises an electrical impedance means coupled to the
piezoelectric film.
6. The system of claim 4 wherein the signal processing means
further comprises an electrical impedance means coupled to the
piezoelectric film, the value of the electrical impedance means
indicating the geometry of the piezoelectric film.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to non-invasive pulse monitors
such as pulse oximeters. In particular, it relates to the detection
of motion transients and the filtering of these transients from the
blood oxygen signals sent to the pulse oximeter.
Photoelectric pulse oximetry is known. Pulse oximeters typically
measure and display various blood flow characteristics including
the blood oxygen saturation of hemoglobin in arterial blood, the
volume of individual blood pulsations supplying the flesh, and the
rate of blood pulsations corresponding to each heartbeat of the
patient. The oximeters pass light through body tissue in a location
where blood perfuses the tissue (i.e. a finger or an ear) and
photoelectrically sense the absorption of light in the tissue. The
amount of light absorbed is then used to calculate the amount of
the blood constituent being measured.
Several different wavelengths of light are simultaneously or nearly
simultaneously transmitted through the body tissue. These
wavelengths are selected based on their absorption by the blood
components being measured. The amount of transmitted light passing
through the tissue will vary in accordance with the changing amount
of blood constituent in the tissue.
An example of a commercially available pulse oximeter is the
Nellcor Incorporated Pulse Oximeter model N-200 (herein "N-200").
The N-200 is a microprocessor controlled device that measures
oxygen saturation of hemoglobin using light from two light emitting
diodes ("LEDs"), one having a discrete frequency of about 660
nanometers in the red light range and the other having a discrete
frequency of about 925 nanometers in the infrared range. The
N-200's microprocessor uses a four-state clock to provide a bipolar
drive current for the two LEDs so that a positive current pulse
drives the infrared LED and a negative current pulse drives the red
LED. This illuminates the two LEDs alternately so that the
transmitted light can be detected by a single photodetector. The
clock uses a high strobing rate, roughly 1,500 Hz, and is
consequently easy to distinguish from other light sources. The
photodetector current changes in response to the red and infrared
light transmitted and is converted to a voltage signal, amplified
and separated by a two-channel synchronous detector--one channel
for processing the red light wave form and the other channel for
processing the infrared light waveform. The separated signals are
filtered to remove the strobing frequency, electrical noise and
ambient noise and then digitized by an analog to digital converter
("ADC"). As used herein, incident light and transmitted light
refers to light generated by the LEDs or other light sources, as
distinguished from ambient or environmental light.
The light source intensity can be adjusted to accommodate
variations in patients' skin color, flesh thickness, hair, blood,
and other variants. The light transmitted is thus modulated by the
absorption of light in the blood pulse, particularly the arterial
blood pulse or pulsatile component. The modulated light signal is
referred to as the plethysmograph waveform, or the optical signal.
The digital representation of the optical signal is referred to as
the digital optical signal. The portion of the digital optical
signal that refers to the pulsatile component is called the optical
pulse.
The detected digital optical signal is processed by the
microprocessor of the N-200 to analyze and identify arterial pulses
and to develop saturation. The microprocessor decides whether or
not to accept a detected pulse as corresponding to an arterial
pulse by comparing the detected pulse against the pulse history. To
be accepted, a detected pule must meet certain predetermined
criteria, including the expected size of the pulse, when the pulse
is expected to occur, and the expected ratio of the red light to
infrared light in the detected optical pulse. Identified individual
optical pulses accepted for processing are used to compute the
oxygen saturation from the ratio of maximum and minimum pulse
levels as seen by the infrared wavelength.
A problem with pulse oximeters is that the plethysmograph signal
and the optically derived pulse rate may be subject to irregular
variants in the blood flow that interfere with the detection of the
blood flow characteristics. For example, when a patient moves,
inertia may cause a slight change in the venous blood volume at the
sensor site. This, in turn, alters the amount of light transmitted
through the blood and the resetting optical pulse signal. These
spurious pulses, called motion artifacts, may cause the oximeter to
process the artifact waveform and provide erroneous data.
It is well known that electrical heart activity occurs
simultaneously with the heartbeat and can be monitored externally
and characterized by an electrocardiogram (`ECG`) waveform. The ECG
waveform comprises a complex waveform having several components
that correspond to electrical heart activity. A QRS component
relates to ventricular heart contraction. The R wave portion of the
QRS component is typically the steepest wave therein, having the
largest amplitude and slope, and may be used for indicating the
onset of cardiovascular activity. The arterial blood pulse flows
mechanically and its appearance in any part of the body typically
follows the R wave of the electrical heart activity by a
determinable period of time that remains essentially constant for a
given patient.
One method to reduce or eliminate the effects of motion artifacts
is to synchronize the ECG signal and the optical pulse signal and
process the two signals to form a composite signal. This composite
signal is then used to measure the level of oxygen saturation. This
method is called ECG synchronization.
In the first stage of synchronization, the optical pulse signal is
filtered to minimize the effects of electronic high frequency
noise, using a low pass filter. Next, the oximeter positions the
newly acquired optical pulse in memory, using the QRS complex as a
reference point for aligning sequential signals. In other words,
when the QRS complex occurs, the oximeter begins processing the
optical pulse data.
In the third stage, the new optical pulse signal is combined with
the composite of the signals that were previously stored in the
memory. Signals are combined using an adjustable weighted algorithm
wherein, when the new composite signal is calculated, the existing
memory contents are weighted more heavily than the new optical
signal pulse.
Finally, the oxygen saturation level is measured from the composite
signal. This determinaton is on the ratios of the maximum and
minimum transmission of red and infrared light. As each sequential
QRS complex and optical pulse signal are acquired, the process of
filtering, positioning, combining and measuring saturation is
repeated. As aperiodic signals such as motion artifacts will not
occur synchronously on the ECG and the detected optical pulse, the
effect of these aperiodic signals is rapidly attenuated.
Another method to detect and reduce the effect of motion artifacts
involves correlating the occurrence of cardiovascular activity with
the detection of arterial pulses by measuring the ECG signal,
detecting the occurrence of the R-wave portion of the ECG signal,
determining the time delay by which an optical pulse in the
detected optical signal follows the R-wave, and using the
determined time delay between the R-wave and the following optical
pulse to evaluate arterial blood flow only when it is likely to
represent a true blood pulse. The measured time delay is used to
determine a time window when, following the occurrence of an
R-wave, the probability of finding an optical pulse corresponding
to a true arterial pulse is high. The time window provides an
additional criterion to be used in accepting or rejecting a
detected pulse as an optical pulse. Any spurious pulses caused by
motion artifacts or noise occurring outside of the correct time
window are typically rejected and are not used to calculate the
amount of blood constituent. Correlating the ECG with the detached
optical pulses thus provides for more reliable measurement of
oxygen saturation.
Other methods to detect and eliminate the effects of patient motion
have been developed. A time-measure of the detected optical signal
waveform containing a plurality of periodic information
corresponding to arterial pulses caused by the patient's heartbeat
and periodic information unrelated to pulsatile flow is collected,
and the collected time measure of information is processed to
obtain enhanced periodic information that is closely related to the
most recent arterial pulsatile blood flow. The time-measure may
comprise a continuous portion of detected optical signals including
a plurality of periodic information from successive heartbeats, or
a plurality of discrete portions of detected optical signals
including a corresponding plurality of periodic information.
By updating the time-measure of information to include the most
recently detected aperiodic information, and processing the updated
measure collectively, an updated enhanced periodic information is
obtained (including the new and historical data) from which
aperiodic information (including any new aperiodic information) is
attenuated. In some embodiments, the updating process includes
subtracting detected signals older than a certain relative time
from the collected time-measure. By collectively processing a
time-measure including successive periodic information to obtain
the enhanced periodic information, and using the enhanced periodic
information as the basis for making oxygen saturation calculations,
the accuracy and reliability of oxygen saturation determinations
can be significantly increased. The time-means may be collectively
processed in either the time domain or the frequency domain.
By synchronizing the occurrence of successive R-waves, it becomes
possible to add the corresponding successive portions of the
detected optical signal together so that the periodic information
(optical pulses) corresponding to the arterial pulse in each
portion will add in phase. The weighted magnitude of the new
periodic information is reinforced by the existence of the weighted
enhanced periodic information at the same time location in
accordance with the degree of synchrony. If the new optical pulse
is identical to the composite pulse then the updated result is a
composite optical pulse having the same magnitude. If the
magnitudes differ, the additive result will differ according to the
relative weights.
As a result of the collected, synchronized additive process, any
aperiodic information that may be present in the portions of the
detected optical signals are also weighted and added to the
weighted composite portion waveform. However, because aperiodic
signals differ in pulse shape, duration, height, and relative time
of occurrence within each portion, and are not synchronous with
heart activity, they do not add in phase. Rather, they add in a
cancelling manner whereby their weighted sum is spread across the
relative time frame of the composite portion.
By processing portions including the periodic information
collectively, aperiodic information is attenuated by the absence of
any corresponding historical aperiodic signal in the prior
composite portion or any subsequent aperiodic signal at that
relative time following heart activity. As the new information can
be given a small weight compared to the absolute weight given the
prior composite, new aperiodic information is quickly and
effectively attenuated and filtered out of the resultant additive
portions.
Although all of the described methods improve the quality of the
pulse oximeter's measurements by reducing the effects of motion
transients and other spurious signals, they provide no independent
indication that motion has occurred. Such an independent
verification of patient motion is useful for pulse oximetry. In
certain cases, it is also possible that an ECG signal will not be
available. In these cases, having an independent motion detection
capability would be essential to detect motion artifacts.
SUMMARY OF THE INVENTION
A preferred embodiment of the present invention comprises a method
and apparatus for minimizing the effect of motion artifacts in
pulse oximetry. Unlike known methods, the present invention derives
a motion detection signal independently of the pulse signal.
Although the present invention will be described relative to its
use in pulse oximetry, its usefulness is not limited to that area
alone.
A preferred embodiment of the present invention will be described
in connection with an adhesive finger sensor for use with a pulse
oximeter. Other sensors may be used, however, without departing
from the scope of the invention.
In an adhesive finger sensor for a pulse oximeter, a strip of
piezoelectric film has been incorporated. The film covers the
nearest movable joint to the sensor; in this example, the joint on
the finger to which the sensor is attached. The change of strain on
the motion sensing element caused by moving the finger to which the
sensor is attached generates a charge within the element, as in a
capacitor. A gain resistor mounted across the motion sensing
element bleeds off the charge, thereby creating a voltage signal
that is proportional to the rate of bending.
By properly processing this voltage signal, motion artifacts can be
detected and their effect on the calculation of blood oxygen
compensated for.
The invention will now be described in detail, with reference to
the figures tested and described below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an assembled sensor according to the present
invention;
FIG. 2 is a cross-section of the sensor shown in FIG. 1;
FIG. 3 is a schematic of the preamplifier used in the present
invention; and
FIG. 4 is a flow chart showing how the present invention processes
motion transient signals.
DESCRIPTION OF THE SPECIFIC EMBODIMENT(S)
A preferred embodiment of the motion detection sensor of the
present invention is shown in FIGS. 1 and 2. Sensor package 10,
which includes light-emitting diodes 13 and 16 and photodetector
15, is for transillumination of a blood perfused portion of flesh
to measure light extinction during transillumination. The sensor is
preferably mounted on a fingertip but any digit or other blood
perfused tissue will work. The sensor conforms to and with the
cutaneous layer of the blood perfused portion of flesh upon which
the sensor is placed. A first end 11 of sensor 10 is disposed on
one side of the flesh to be transilluminated and a second end 18 is
disposed on the opposite and opposed side of the flesh to be
transilluminated.
When the sensor is adhesively fastened, the effect of the light
source and photodetector being integrated into the adhesive
fastener is that they become, in effect, a part of the skin. The
resulting device is resistant to accidental removal and avoids
constriction of blood vessels. Most importantly, the low mass of
the sensor itself and its conformance to the skin prevents motion
and the possible resulting contact interruption between the light
source, photodetector and flesh.
In the present invention, as illustrated in FIGS. 1 and 2, the
dimensions of the butterfly-shaped bandage containing the sensor
are such that the butterfly "wings" (ends 11 and 18) do not extend
beyond the first joint of the patient's finger when the sensor is
attached to a patient. Bandage layer 21 is preferably an adhesive
cotton elastic material which completely covers opaque white
polypropylene layer 14. Holes are formed in opaque, adhesive coated
polypropylene layer 14 for the optical components. A clear,
double-coated 0.003 thick polyethylene layer 12 covers these
holes.
The LEDs 13 and 16, as well as photodetector 15 are placed beneath
layer 12. Photodetector 15 is mounted on lead frame package 26 and
is surrounded by Faraday shield 17. The LEDs, photodetector and
Faraday shield are all coupled to the pulse oximeter by means of
leads running through cable 23. LEDs 13/16 are commercially
available and are mounted in a lead frame package 29. The red
wavelength LED generates at least 0.85 milliwatts and the I.R. LED
generates 1.45 milliwatts of power. In an alternate embodiment, the
lead frame packages 26 and 29, photodetector 15 and LEDs 13 and 16
are mounted on a flexible substrate 25. In the preferred
embodiment, opaque layer 14 and clear layer 12 are peanut-shaped to
provide adequate coverage of the optical components, wires and
motion sensor. The peanut shape also provides sufficient surface
area to adhere to the butterfly without subsequent delamination and
minimizes assembly time.
Motion sensing element 19 is a strip of piezoelectric film placed
between the optical components and the bandage layer 21. In the
preferred embodiment, the sensing element is made from KYNAR film,
a product of Atochem, Inc. The motion sensing element extends
across the sensor head from one butterfly wing end 18 through and
beyond the other end 11, into a tab 27 that, in the preferred
embodiment, is disposed over the first joint of a finger, on the
dorsal side, when the sensor is applied to the patient.
The change of strain on the motion sensing element (such as by
bending the film strip) generates a charge within the element, as
in a capacitor. A gain resistor mounted across the motion sensing
element bleeds off the charge, thereby creating a voltage signal
that is proportional to the rate of bending. The size of the gain
resistor may be varied to permit differently dimensioned oximeter
sensors (with differently dimensioned motion detection elements) to
be used with the same oximeter. In the preferred embodiment, the
gain resistor is mounted in the sensor connector.
An electrical cable 23 provides the LED driving current and returns
photodetector 15 and motion sensing element 19 signals to the
oximeter. In the preferred embodiment, the cable contains three
shielded, twisted pairs of conductors, one pair each for the
detector, the emitters and the motion sensing element. The cable's
inner shield is coupled to the photodetector's Faraday shield. Both
the outer and inner cable shields are tied to analog ground. All
wires are terminated in the sensor connector.
In the preferred embodiment, an emitter coding resistor is included
in the sensor connector. As is more fully explained in U.S. Pat.
No. 4,621,643, the value of the coding resistor is related to the
operating wavelengths of the emitters. The oximeter reads the value
of this resistor to determine which coefficients to use in the
saturation calculation.
In the preferred embodiment, the sensor connector is plugged into
the front end of a custom preamplifier. The preamplifier may be
external to the oximeter or incorporated within the oximeter.
As shown in FIG. 3 preamplifier 100 comprises a first section 101
to amplify the photodetector signal used to compute oxygen
saturation and a second section 151 to condition the motion
detector's output.
First section 101 comprises a differential input amplifier with an
approximate gain of 1 million. This requires the sensor to be
configured in a differential mode with shielded twisted pair
conductors. No offset voltage is provided for dynamic range
improvement but could be added. The output of the differential
amplifier is transmitted to the pulse oximeter in known
fashion.
As stated previously, KYNAR piezoelectric film element 19 can be
modeled as a capacitor. When a strain is placed on the film, a
charge is produced. The output of the film is proportional to the
rate of change of the strain and it is A.C. coupled. To use this
charge, a resistor 152 needs to be coupled in parallel with the
film. The value of this resistor affects the voltage sensitivity of
the film, which simply means that different sensor geometries need
to be tuned with different resistors.
The voltage signal from the film/resistor combination is then
passed through a unity gain, second order Butterworth filter 155
with a cut-off frequency of 10 Hz to reject line noise pickup. The
band-limited signal is then amplified in amplifier 160 by a factor
of 33,000 along with an inserted (adjustable) offset of 2.1 volts.
The selection of the gain is arbitrary, based on obtaining
"reasonable" output for typical motions. The offset was added to
place the A.C. coupled output approximately in the middle of a 5
volt ADC input range.
In the preferred embodiment, the N-200 is modified to receive the
conditioned motion signal through an unused channel of an ADC. The
optical pulse signal is sampled at 57 hz, the ECG signal at 200 hz,
and the motion signal at about 57 hz. The N-200 software is
modified to read this additional ADC channel and process it along
with the optical and ECG information. Collection of the optical and
ECG signals is not changed.
Referring now to FIG. 4, the oximeter detects the presence of
motion by subtracting from the baseline signal of the motion signal
at step 203, after it has been conditioned to remove background
noise, taking the absolute value of the result and entering a
"motion present" state at step 207 whenever the processed signal
passes a fixed threshold as determined at step 205. In the
preferred embodiment, the optimum threshold was determined
empirically to be 1.22 millivolts. The oximeter leaves the "motion
present" state 1.5 seconds after the processed signal falls below
the threshold.
Entering a "motion present" state at step 207 changes the way the
optical signals are processed and, therefore, the way blood oxygen
saturation is calculated. Outside of the "motion present" state
(step 213), the oximeter calculates blood oxygen saturation in any
known appropriate manner. In the preferred embodiment, the oximeter
maintains a history (step 221) consisting of the mean values over
four consecutive pulses of three parameters as part of the
saturation calculation algorithm: the period between successive
optical minima, the IR optical pulse amplitudes, and the
"ratio-of-ratios". The period and amplitude information is
displayed by the oximeter. "Ratio of ratios" is used in the
saturation calculation and is defined as follows: ##EQU1## Incoming
pulses are checked against the history, and pulses are rejected if
they are outside the permitted limits of variation (step 215). The
first four pulses rejected for variation excess are not placed into
the pulse histories (step 217 and 219). Once four pulses are
rejected for this reason, subsequent pulses are placed into the
history at step 221 to permit the history to reflect changing
physiological conditions. If the pulse is accepted, a time-out
clock is reset. The time-out clock normally sounds an alarm if no
qualified pulse is detected within 15-20 seconds.
Before using the ratio-of-ratios in the saturation calculation, it
is filtered as follows:
where 1.ltoreq.N.ltoreq.255 and N varies according to pulse rate
and amplitude. For the first 5 pulses after locking onto the
optical pulse, use N=255. After the first 5 pulses, calculate N for
each pulse. The initial N varies depending upon the type and
physiology of the patient. If locked on ECG, multiply the result by
3. If the rate is greater than 100, divide the result by 2. If the
average IR amplitude is small, divide the result by 2. Filter the
result against the previous result using a 1/2 old, 1/2 new filter.
The final answer becomes the new N. Note that as N rises, the
effective filtering decreases.
When the oximeter enters the "motion present" state (step 205),
pulses which do not conform to the "history" accumulated prior to
entering the "motion present" state are not accepted. This prevents
the oximeter from mistakenly accepting false pulses caused by
motion artifacts which pass other criterial checks employed by the
oximeter after the first four bad pulses have gone by.
Additionally, it prevents the oximeter from building up a history
consisting of false pulses caused by motion artifact which would
then prevent the N-200 from accepting good pulses once the motion
artifact ceases.
Also, the oximeter uses a higher "N" value in the filtered ratio
calculation for accepted pulses (step 207). This change permits the
oximeter to use tighter filtering on data during the motion present
state, while allowing the instrument to return its normal response
time when motion is not present. Finally, the oximeter employs a 45
second pulse time-out period (step 207), as compared to the 15-20
second time out used when motion is not present (step 213) before
triggering an alarm indicative of loss of pulse in the patient.
The foregoing description provides a full and complete disclosure
of the preferred embodiments of the invention. Various
modifications, alternate constructions, and equivalents may be
employed without departing from the true spirit and scope of the
invention. For example, although only the use of a piezoelectric
film to provide motion detection has been described herein, other
motion detection means such as accelerometers, or stain gauges
could be substituted without changing the substance of this
application. Therefore, the above description and illustrations
should not be construed as limiting the scope of the invention
which is defined by the appended claims.
* * * * *